Bisulfite, a common food preservative, has been previously reported to be mutagenic in a number of cellular systems and also in phage T4. Because phage T4 employs 5-hydroxymethylcytosine instead of ordinary cytosine in its DNA, however, and because 5HMC is chemically far more resistant to the mutagenically significant action of bisulfite normally observed with cytosine itself, the action of bisulfite upon T4 should be critically re-examined. Preliminary experiments have demonstrated a virtually complete nonreproducibility of the earlier T4 report in several major respects. Future work will explore the mutagenicity of bisulfite on T4 containing cytosine in its DNA instead of 5HMC.